Add how to migrate to ODK .md and link to it

[StroemPhi]

This PR adds the odk-migrate-to-odk.md to the docs/howto folder to provide a How To article for the migration of an existing ontology repository to ODK, based on this draft https://docs.google.com/document/d/10i76WqZ8EbnW4ta7BBNqTOYH2tLx51QDzhWF63-80uY/.

CC:
@matentzn
@cmungall

[gouttegd]

Overall looks good to me, but I believe there are issues with the indenting of keys in the ODK config file shown as example. This must be fixed as people are likely going to be copy-paste that example to “bootstrap” their own migration effort.

[StroemPhi]

Overall looks good to me, but I believe there are issues with the indenting of keys in the ODK config file shown as example. This must be fixed as people are likely going to be copy-paste that example to “bootstrap” their own migration effort.

Thank you for the review. I've addressed all of your points and fixed some typos/improved some wording (9d8f1af).